1. Field of the Invention
The invention relates to an image forming apparatus which is usable for a copying machine, a printer, a plotter, a facsimile, or similar printing apparatus.
2. Description of Related Art
A conventionally known image forming apparatus is disclosed in U.S. Pat. No. 3,689,935. The apparatus uses an electrode having plural opening portions (hereinafter referred to as "apertures") and a voltage is applied to the electrode in accordance with image data to control passage of toner particles through the apertures, whereby an image is formed on a supporter (image receiving medium) with the passed toner particles.
The image forming apparatus includes an aperture electrode unit comprising an insulating flat plate, a reference electrode formed continuously on one surface of the flat plate, plural control electrodes which are formed on the other surface of the flat plate and electrically insulated from one another and at least one row of apertures, each aperture formed in correspondence with a control electrode so as to penetrate through the flat plate, the reference electrode and the corresponding control electrode; means for selectively applying a voltage across the reference electrode and the control electrodes; means for supplying charged toner particles so that the flow of the toner particles passed through the apertures is modulated in accordance with the applied voltage, and means for moving a supporter and the aperture electrode unit relative to one another to position the supporter in a particle flow passage.
Further, U.S. Pat. Nos. 4,743,926, 4,755,837, 4,780,733, and 4,814,796 disclose image forming apparatuses having an aperture electrode unit disposed so that control electrodes face a supporter and a reference electrode faces a toner supply side.
On the other hand, U.S. Pat. No. 4,912,489 discloses an aperture electrode unit disposed so that the reference electrode faces the supporter and the control electrodes face the toner supply side. The specification of U.S. Pat. No. 4,912,489 discloses that a voltage applied to the control electrodes at an off-time can be reduced to about a quarter of that of the image forming apparatus as disclosed in the above U.S. Patents.
The term "off-time" means a time when no toner particle is attached onto the supporter, that is, when a blank portion of an image is formed on the supporter. Conversely, the term "on-time" means a time when a toner image is formed on the supporter.
However, the conventional image forming apparatus as described above has the following problem during image recording. In order to form continuous dots in a direction perpendicular to the sheet feeding direction, in some cases the neighboring apertures are arranged in an alternating pattern such that one row of apertures covers the gaps of an adjacent row so the rows are overlapped with one another in the sheet feeding direction. However, an image formed by the aperture electrode unit as structured above has a problem that the print characteristic of the dots which are formed by the respective neighboring apertures arranged in the alternating pattern is varied in accordance with each aperture. That is, a dot to be formed by an aperture is normally formed, however, a dot to be formed by an aperture adjacent to the former aperture is formed densely in print density, thereby inducing fog in the formed image. Accordingly, there occurs a problem that the dots formed by the neighboring apertures are different from each other in print density, so that it is very difficult to perform a stable recording operation for all arrays of apertures.
As a countermeasure for the above problem, it has been considered that the apertures be not arranged in a zigzag form, but that the apertures be aligned with one another in a direction perpendicular to the sheet feeding direction. However, in order to enable the continuous dots to be recorded in the direction perpendicular to the sheet feeding direction, the respective apertures are required to be continuous with each other. This requirement means that one large aperture must be formed in the longitudinal direction, and this is practically impossible.